The goal of accessing data from a drill string has been expressed for more than half a century. As exploration and drilling technology has improved, this goal has become more important in the industry for successful oil, gas, and geothermal well exploration and production. For example, to take advantage of advances in the design of various tools and techniques for oil and gas exploration, it would be beneficial to have real time data, such as temperature, pressure, inclination, salinity, etc., and to be able to send control signals to tools downhole. A number of attempts have been made to devise a successful system for accessing such drill string data and for communicating with tools downhole. These systems can be broken down into four general categories.
The first category includes systems that record data downhole in a module that is periodically retrieved, typically when the drill string is lifted from the hole to change drill bits or the like. Examples of such systems are disclosed in the following U.S. Pat. No. 3,713,334 issued to Vann, et al., U.S. Pat. No. 4,661,932 issued to Howard, et al., and U.S. Pat. No. 4,660,638 issued to Yates. Naturally, these systems have the disadvantage that the data is not available to the drill operator in real time.
A second category includes systems that use pressure impulses transmitted through the drilling fluid as a means for data communication. For example, see U.S. Pat. No. 3,713,089 issued to Clacomb. A chief drawback to this mud pulse system is that the data rate is slow, i.e. less than 10 baud. In spite of the limited bandwidth, it is believed that this mud pulse system is the most common real time data transmission system currently in commercial use.
A third category includes systems that use a combination of electrical and magnetic principles. In particular, such systems have an electrical conductor running the length of the drill pipe, and then convert the electrical signal into a corresponding magnetic field at one end. This magnetic field is passed to the adjacent drill pipe and then converted to back to an electrical signal. An example of such a system is shown in U.S. Pat. No. 6,717,501 issued to Hall et al., and incorporated herein by reference. In the Hall system, each tubular has an inductive coil disposed at each end. An electrical conductor connects the inductive coils within each tubular. When the tubulars are made-up in a string, the inductive coils of each tubular are in sufficiently close proximity that the magnetic fields overlap to allow data transmission across the connection between the tubulars. Because of a partial loss of the signal between each tubular, the commercial embodiment of Hall, which is marketed by Grant Prideco (Houston, Tex.) as Intellipipe™, uses repeater stations positioned at regular intervals in the drill string to boost the signal.
A fourth category includes systems that transmit data along an electrical conductor that is integrated into the drill string. Examples of such systems are disclosed in U.S. Pat. No. 3,879,097 issued to Oertle; U.S. Pat. No. 4,445,734 issued to Cunningham, and U.S. Pat. No. 4,953,636 issued to Mohn. Each of these systems includes forming direct electrical connections between each tubular.
An early system using electrical connections for transmitting telemetry data is disclosed in U.S. Pat. No. 3,518,608 issued to Papadopoulos in 1970, and incorporated herein by reference. That system uses strips of conductors (referred to as “contacts”) mounted with an insulating epoxy on a modified portion of the threads on the connection. Papadopoulos discloses the use of threads having a substantially V-shaped form that are modified by topping off (i.e. removal of upper portion of the thread) the crest on the pin thread and cutting a groove in the root of the box thread where the contacts are attached. Papadopoulos discloses that both the male and female contacts are at least one full thread in length (i.e. one pitch). When the connection is made-up, the conductor strips come into contact and are able to transmit an electrical signal across the connection. To ensure electrical contact, Papadopoulos discloses that the female copper contact should be slightly oversized. If wear of the conductors prevents good electrical contact, Papadopoulos discloses that coating the face of the male contact with a mixture of epoxy cement and copper dust can provide the electrical contact. Papadopoulos also discloses that the root space of all the pin threads should be free to maintain a desired communication of fluid between the inside of the drill pipe, through the threads, and to the annular space above the threads. As a result, no fluid pressure gradient can exist across the electrical contact.
Because a drill string can include hundreds of sections of tubulars, electrical connectors must be provided between each tubular section to carry the data signal. Connector reliability is critical because the failure of any one connector will prevent data transmission. A challenge to connector reliability is that the downhole environment is quite harsh. The drilling fluid pumped through the drill string is abrasive and typically has a high salt content. In addition, the downhole environment typically involves high pressures and temperatures, and the drill string is subjected to large stresses from tension, compression, bending, and torque. Surface handling of tubulars also challenges connector reliability. Heavy grease is typically applied at the joints between tubular sections. The connections are “stabbed” together, and then made-up. During the stabbing, electrical contactors are at risk of damage from impacts.
If a reliable transmission system using an electrical signal is achieved, the higher data transmission rates could provide a wealth of information during drilling operations and later during the production of hydrocarbons. Advances in sensors allow for valuable data to be gathered about performance during drilling, the formation surrounding the wellbore, and conditions in the wellbore. The value of that data would increase if it was made available in real time. What is still needed is a connection for a tubular that allows reliable data transmission despite the many challenges to connector reliability present in downhole applications.